1. The field of the invention comprises thread-forming tools.
2. So far as is known, the invention is new, although the principle of forming threads, as used herein, is old. Thread forming or thread rolling is basically a cold forging process in which a thread is formed by displacement of material. The advantages of the process are well known, such as accuracy as required for precision threads, uniformity, smoothness, material economy in that the blank size is closer to the finished product size, thread strength, high production rate, and, of special importance, the absence of cuttings or chips which otherwise would be difficult to remove. The process is generally limited to external threading, using a pair of flat-faced reciprocating dies to form threads on, say, a bolt blank disposed between the dies, or using a pair of cylindrical dies. In a few applications an external thread on a thin-walled tube is rolled with a matching mandrel to form an internal thread as well. In one recently patented system, a copper tube is described as being internally threaded by means of a rotating shaft disposed inside the tube, the shaft having a tool holder mounted in its end which carries a fixedly secured cutter in the form of a short solid rod having a bevelled end whose edges do the cutting. As the shaft rotates, so does the cutter, and the tube is moved linearly past the cutter so that a groove may be cut in the inner surface of the tube. As will be described, the invention is unlike the foregoing system in not using a fixed, non-rotating, shaft-mounted cutter, but rather using housing-mounted rotatable thread-forming means that is radially movable toward and from the internal pipe surface that is to be threaded, and in having means for controlling such radial movement of the thread-forming means. A significant advantage may thus be apparent, for by moving the thread former radially away from the said surface, the tool can be inserted into the pipe to any point along its internal length, and any part of such length can be threaded by moving the thread former radially into operative contact with the said surface. Such selective internal threading cannot be done with the described patented system or with any other process known to applicant. Other advantages will be apparent as the description proceeds.